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SNTP: Implement round-trip delay compensation

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Introduce a new configuration define SNTP_COMP_ROUNDTRIP to compensate
for network round-trip delays when setting the system clock from SNTP.
Note that this feature requires compiler support for 64-bit arithmetic.
This commit is contained in:
Daniel Elstner
2017-03-25 16:57:51 +01:00
committed by goldsimon
parent cc77b308a4
commit 751ee7a534
2 changed files with 74 additions and 4 deletions
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27
src/include/lwip/apps/sntp_opts.h
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@@ -109,6 +109,30 @@
#define SNTP_CHECK_RESPONSE 0
#endif
/** Enable round-trip delay compensation.
* Compensate for the round-trip delay by calculating the clock offset from
* the originate, receive, transmit and destination timestamps, as per RFC.
*
* The calculation requires compiler support for 64-bit integers. Also, either
* SNTP_SET_SYSTEM_TIME_US or SNTP_SET_SYSTEM_TIME_NTP has to be implemented
* for setting the system clock with sub-second precision. Likewise, either
* SNTP_GET_SYSTEM_TIME or SNTP_GET_SYSTEM_TIME_NTP needs to be implemented
* with sub-second precision.
*
* Although not strictly required, it makes sense to combine this option with
* SNTP_CHECK_RESPONSE >= 2 for sanity-checking of the received timestamps.
* Also, in order for the round-trip calculation to work, the the difference
* between the local clock and the NTP server clock must not be larger than
* about 34 years. If that limit is exceeded, the implementation will fall back
* to setting the clock without compensation. In order to ensure that the local
* clock is always within the permitted range for compensation, even at first
* try, it may be necessary to store at least the current year in non-volatile
* memory.
*/
#if !defined SNTP_COMP_ROUNDTRIP || defined __DOXYGEN__
#define SNTP_COMP_ROUNDTRIP 0
#endif
/** According to the RFC, this shall be a random delay
* between 1 and 5 minutes (in milliseconds) to prevent load peaks.
* This can be defined to a random generation function,
@@ -142,7 +166,8 @@
#endif
/** SNTP macro to get system time, used with SNTP_CHECK_RESPONSE >= 2
* to send in request and compare in response.
* to send in request and compare in response. Also used for round-trip
* delay compensation if SNTP_COMP_ROUNDTRIP != 0.
* Alternatively, define SNTP_GET_SYSTEM_TIME_NTP(sec, frac) in order to
* work with native NTP timestamps instead.
*/